1. Is the speed true, possibly, so long as the various estimators suggest this, and the line length fits this sort of speed. The £5 per month, ensure that is for the full term of the contract and not a 3 months at £5 rising to standard £10 thereafter.
2. Not sure, worst case they extend contract.
3. Free connection is a common offer, so not that amazing.
Basically you need the offer in an email for your records and proof, and how do you know it was TalkTalk, and not just an affiliate doing a very good sales pitch on you?